Generally, heading (flowering) of rice is promoted by short-day conditions and delayed by long-day conditions. Among known cultivars, typically those from Kyushu and the south of Mainland Japan have strong photoperiod sensitivity whereas cultivars from the Tohoku district or Hokkaido show complete loss of such sensitivity or have extremely weak photoperiod sensitivity. Rice plants that lack the photoperiod sensitivity have a characteristic to flower after a certain length of growth period, and the heading date of the plant does not change with changes in photoperiod. Adaptation of rice plants in particular locations and seasons drastically changes in accordance with the existence of photoperiod sensitivity in the plant. Thus, modification of photoperiod sensitivity in rice is an important aspect of rice breeding.
In conventional breeding programs, the alteration of the heading date of rice is achieved through methods involving: (1) selection of early maturing varieties or late varieties by crossing; and (2) mutagenesis by radiation and chemicals; and so on. However, such breeding programs require long periods of time to be successful, and bear other problems, such as unpredictability of the degree or direction of the variation in the progeny.
The term “photoperiod sensitivity gene” is a generic term for genes that enhance the rice photoperiod sensitivity in the field of rice genetics. The existence of several photoperiod sensitivity genes has been observed to be inherent in mutants and cultivars, and photoperiod sensitivity genes are suggested to exist on loci, for example, such as Sel locus (chromosome 6; Yokoo and Fujimaki (1971) Japan. J. Breed. 21:35–39), E1 locus (chromosome 7; Tsai, K. H. (1976) Jpn. J. Genet. 51: 115–128; Okumoto, Y. et al. (1992) Jpn. J. Breed. 42: 415–429), E2 locus (unknown), E3 locus (chromosome 3; Okumoto et al. Japanese Society of Breeding, 91st lecture, Japanese Journal of Breeding 47(Suppl. 1): 31), and so on (Yamagata et al. (1986) In Rice Genetics, International Rice Research Institute, Manilla, pp351–359).
When a photoperiod sensitivity gene of rice or a gene regulating the heading of rice under the control of the photoperiod sensitivity gene is isolated, introduction of the gene into any desired rice cultivar by a transformation method makes it possible to control the heading time of the rice cultivar. Furthermore, the flowering time of various plants can be controlled by utilizing genes of other plants corresponding to such rice photoperiod sensitivity gene. This kind of breeding method can be said to be extremely advantageous compared to conventional methods in the aspect of convenience and reliability.